In a conventional wireless communications system, a mobile station (cellular telephone, portable computer, etc.) is served by a network of base stations. Such base stations serve as the communication relay station for mobile stations. Accordingly, a mobile station must be in wireless communication with at least one base station whenever the mobile station is turned on in order to communicate with the other components of a communications system. Mobile stations sometimes move out of a region served by one base station and into a region served by another base station. Base stations note this fact, and "handoff" communication from the first base station to the second. It is common for a mobile station to be in communication with both the first and second base station for periods of time. A mobile station that is in communication with more than one base station is said to be in "soft handoff". In some cases, a mobile station will be in soft handoff with more than two base stations at any one time.
Soft handoff is desirable because it reduces dropped calls. In addition, soft handoff allows the mobile unit to receive the same information from more than one source and to use all of this received information (or energy) to assist in decoding the information that is being transmitted to the mobile station by each base station. Using information transmitted from more than one base station means that the power level that is required from any one base station is reduced.
One type of wireless communication system is known as Code Division Multiple Access (CDMA). CDMA systems offer greater capacity than other systems. That is, the number of channels of information that can be communicated concurrently is greater in CDMA systems than in other systems, such as a time division multiple access (TDMA) system or a frequency division multiple access (FDMA) system.
In CDMA systems in which both voice and data are being communicated, base stations transmit to as many mobile stations as are in the coverage area for that base station on the same frequency at the same time. In addition, each such base station transmits at the same frequency as every other base station in the network. Signals transmitted to a particular mobile station are distinguishable from signals transmitted to other mobile stations only by the fact that they are transmitted using different codes. In contrast, in a TDMA system, transmissions to a first mobile station are sent during a first period of time and transmissions to a second mobile station are sent during a second, non-overlapping period of time. In an FDMA system, transmissions to a first mobile station are transmitted on a first frequency and transmissions to a second mobile station are transmitted on a second frequency. Because a CDMA receiver can receive more than one channel at a time while tuned a single frequency, a CDMA receiver can more conveniently perform soft handoff then can a TDMA receiver or an FDMA receiver.
While CDMA systems have the advantage of being ideally suited for soft handoff, signals transmitted to a first mobile station using a first code appear as noise to a second mobile station attempting to receive signals transmitted to the second mobile station using a second code. This interference is preferably minimized by making the codes assigned to signals transmitted from a base station orthogonal with codes assigned to all other signals being transmitted by that base station. However, codes used with signals that are transmitted by a first base station can not be made orthogonal with codes used with signals transmitted by a second base station. Therefore, base stations carefully regulate the amount of power used to transmit signals to mobile stations. The power must be high enough to get the signal through, but the power is preferably no higher than necessary, since additional power appears to other mobile stations as additional interference and reduces the number of mobile stations that can be served.
Because conventional CDMA communication systems must handle both voice and data, certain performance requirements must be met. One such requirement is that the delay between the time the information is transmitted from one end of the communication system until the time the information is received at the other end of the communication system must be relatively short. That is, when two people are talking, any perceptible delay between the time words are spoken at one end of the line and the time those words are heard at the other end of the line would be annoying to both the speaker and the listener.
In contrast, many data communication systems can tolerate relatively long delays between the time information is sent and that information is received. CDMA systems have recently been proposed that take advantage of the fact that relatively long delays can be tolerated in systems designed to handle only data. Such systems are referred to herein as high data rate (HDR) systems. In HDR systems, a base station is dedicated to communicating with only one mobile station at any one time. The capacity advantages of CDMA are realized with HDR systems. However, it may be difficult or undesirable to perform soft handoff due to the following reasons. First, the transmissions from a base station in an HDR system are all directed to one mobile station at any particular time. Therefore, while the number of code channels being transmitted from an HDR base station is essentially the same, all of the code channels are intended to be received by one mobile station at any one time. As a result, it is complex to coordinate the times of transmission between two base stations in order to allow soft handoff between the two base stations. Second, in order to perform soft handoff, it is necessary to distribute the same data between more than one base station. This will greatly increase the amount of data to be transferred between base stations, especially for high data rate applications. Third, system capacity increases if the mobile unit can always connect to the best serving base station instead of using soft handoff, assuming that the channel condition is relatively static, as is likely for an HDR system. This is true, since an HDR base station typically transmits at maximum power to allow the best data rate. That is, the rate at which data can be transmitted is directly proportional to the amount of power received. Therefore, in order to maximize the data rate, the maximum power is transmitted. However, this increases the amount of interference a first base station contributes to the signals received by mobile stations attempting to receive signals from a second base station.
Accordingly, there is a need for a method and apparatus for reducing the amount of interference that a first base station contributes to mobile stations attempting to receive signals from one or more other base stations.